The possibility of igniting energy-generating fusion by impact, essentially by firing a bullet into a target at extreme speed ˜1000 km/sec, was studied in the 1960s and 1970s. However, firing a bullet massing ˜10 mg to that speed is extremely demanding. The most plausible method, then and now, is accelerating a charged bullet in a modified particle accelerator. Unfortunately the charge that can be placed on a macroscopic body is limited by self-repulsion, tending to cause both burst-apart and field effect evaporation of atoms from the surface. The acceleration and vibration a macroscopic body can withstand is also a limiting factor. Even the most optimistic estimates, using an ideal diamond as a ‘bullet’, put the necessary accelerator length at >100 km. An alternative is to use a cluster of microparticles (herein also referred to as particles or pellets) or heavy ions in place of a single bullet, which would reduce the accelerator length. Unfortunately multiple problems exist with this strategy, including the very high accelerator power that would be needed.
However, a fusion reactor for use in space can have design parameters that would be impracticable on Earth. If an extremely long vacuum gap is available, there is time for a stream of microparticles fired from a linear accelerator of relatively modest power to catch up into a cluster if given a small speed differential, with the first particles launched travelling the slowest and the last the most rapidly: the accelerator power needed scales inversely as the size of vacuum gap.